The present invention relates generally to materials that comprise a redox-active moiety and which are selectively reversibly switchable between a reduced state and typically one oxidized state (i.e., redox-switchable). The redox state of the material may be switched chemically (e.g., oxidizing or reducing agents), electrochemically (e.g., by contact with electrodes, or use of redox-active reagents) and/or photochemically. Materials of interest include solid materials in which a redox-active moiety is adsorbed, bonded or both to a surface of the solid so that switching of the redox state of the redox-active moiety affects the properties of that surface, e.g. charge, and/or or affects adsorption or bonding to that surface. Of particular interest are solids incorporating a photoactive semiconductor, preferably a photoactive oxide semiconductor, and a redox-active moiety from which electron-hole pairs can be generated on irradiation to switch redox state.
Redox switchable materials in general have application as redox switches (e.g., selectively changing the redox state or charge of an environment), in sensors and other analytic devices or methods, for selective removal, release or transport of ions, and as catalysts. Such materials may also find application in the control of self-assembly related to molecular devices (Muraoka, 2002 and references therein).
A material whose redox or charge state can be “switched” by external triggers such as light, electricity, or chemical potential has more specific application for use in extraction of solutes from solution; in electrochromic materials, whose optical properties (e.g., color, reflectivity, transparency) change on application of an electric field; in reusable writing media or data storage applications; and in electrocatalysis in which electricity is employed to drive chemical reactions, e.g. to carry out more efficient syntheses of valuable products or to store electricity by making fuel. Photo redox-switchable materials can additionally be employed generally in solar energy applications for storage of light energy in chemical forms for making fuel.
The extraction of species from solution (herein referred to generally as solutes) by adsorption to a solid is well known, as exemplified by the use of ion-exchange resins and zeolites. Furthermore, the specific “recognition” of particular solutes by other molecules, which bind to the solutes preferentially due to complementary fitting of shape, charge, polarizability, etc. has been the subject of extensive research over the last few decades. Macrocyclic compounds, such as crown ethers and cryptands, among others, are examples of compounds that are now being applied as selective adsorbers or binding agents to the selective extraction of solutes from aqueous solution (e.g., Izatt et al., 1991, 1996; Hankins et al., 1996).
These extraction methods, however, suffer from an “elution problem”. Once the adsorber or binding agent is saturated with the extracted solute, release or removal of the solute from the adsorber or binding agent to recover the solute and/or reuse the adsorber or binding agent can be difficult and expensive. This is particularly true when adsorption or binding is highly selectivity. The elution of the solute, and concomittant regeneration of the adsorber, is typically carried out under extreme chemical conditions. In commercial gold hydrometallurgy, for example, aurocyanate adsorbed onto activated carbon must be removed by elution with strong basic solution. Similarly, the release of heavy metals that have been extracted by crown ethers tethered to silica requires elution in strong acid (e.g., ˜1 M HCl; Izatt et al., 1991). Even the regeneration of the ion-exchange resin in an ordinary household water-softener requires flushing with concentrated NaCl brine. Release of solute and regeneration of the adsorber or binding agent can be expensive and/or wasteful in application of energy or materials. In applications to water purification, the result of use of such selective adsorbers or binding agents is often the generation of a greater volume of aqueous waste than the volume of water purified. While such costs and/or waste can be tolerated in particular applications, from a global perspective such purification technologies hinder rather than help the goal of water purification.
Materials in which adsorption and/or binding affinity can be selectively changed would provide improved adsorber and or binding agents. Redox-switchable materials which also incorporate a selective complexing or binding moiety and in which the complexing or binding affinity is affected by redox state would provide selectively switchable adsorber or binding agents useful for overcoming the elution problem discussed above.
Much research has been described on homogeneous, solution-based systems involving redox-activated binding, e.g., involving functionalized ferrocenes. See, for example, Allgeier, 1997; Beer, 1998; Beer, 1996; Beer, 1995; Beer, 1994; Beer, 1993; Beer, Chem. Commun., 1993; Chen, 1995; De Santis, 1992; Hall, 1997; Hall, 1993; Kaifer, 1996; Lloris, 1998; Plenio, 1997; Plenio, Chemische Berichte, 1997; Plenio, 1995; and Su, 1999.
Certain redox-switchable materials exhibit binding affinity that is a function of the redox state. For example, the “ferrocene cryptand” molecule 1 (1,1′-[(1,7-dioxa-4,10-diazacyclododecane-4,10-diyl)diethoxy]-3,3′,4,4′-tetraphenylferrocene) displays selective, redox-switchable binding. The ferrocene moiety undergoes reversible one-electron oxidation, while the cryptand moiety (the nitrogen- and oxygen-containing ring) binds selectively to cations that fit within the ring. This binding is disrupted when the ferrocene is switched into the oxidized state, because of electrostatic repulsion between the bound cation and the positive charge on the Fe atom.

The applications that have been proposed for these homogeneous systems are in sensors for detecting small quantities of solutes, in which binding to the ferrocene would cause a measurable change in potential. Redox-active species are not adsorbed or bonded to solid surfaces. Neither oxidation nor reduction in these systems is reported to be light- or electrically driven. Homogeneous, solution-based redox-activated binding systems have been reported for use in extraction or separation, for example Clark, 1999; Clark, 1996; Strauss, 1999; and Tendero, 1996. For example, Saito, 1986 report the use of a ferrocene functionalized crown ether for electrochemical ion transport. Redox switching is done chemically, not photochemically or electrochemically. Chambliss, 1998 report the use of modified ferrocenes sorbed (no covalent bonding was reported) to silica for recyclable anion-exchange materials.
The attachment of redox-active moieties, such as modified ferrocenes, to electrode surfaces have been reported. See: Albagli, 1993; Anicet, 1998; Audebert, 1996; Blonder, 1996, Bruening, 1997; Bu, 1995; Chao, 1983; Chen, 1994; Ching, 1995; Ching, 1994; Di Gleria, 1992; Hale, 1991; Moutet, 1996; Schuhmann, 1991; and Wang, 1995. Most of these reports have been directed toward the detection and analysis of specific solutes, particularly for biological applications where very small amounts of solutes are involved. Release of solutes from electrodes in such applications is not a major concern. The ferrocene/ferricenium couple has also been widely used for an electrochemical reference standard e.g., Bashkin, 1990.
In specific embodiments, this invention relates to materials in which changes in adsorption or binding affinity are photo-induced. Specifically, the invention relates to redox-switchable materials in which photo-induced redox-switching is mediated by a photoactive semiconductor.
A number of systems involving photoswitchable binding of a solute species have been described. These are not semiconductor based, but utilize photoactive organic molecules of various sorts in which the mechanism of photoactivation (e.g., photoisomerization, photocleaving) is very different. Homogenous systems involving photoactive species in solution not affixed to a surface have been reported. Reports include photoactivated binding to macrocyclic compounds including crown ethers and cryptands. See: Akabori, 1995; Al'fimov, 1991; Barrett, 1995; Blank, 1981; Fuerstner, 1996; Irie, 1985; Kimura, 1997; Marquis, 1998; Martin, 1996; Shinkai, 1996; Shinkai, 1987; Shinkai, 1982; Shinkai, 1983; Stauffer, 1997; and Tucker, 1997. Insofar as potential applications have described they are generally directed toward sensing. Research has also focused on photosystems that are models of biological systems.
Systems involving photoswitchable binding of a solute species directed toward extraction, separation or ion transport have been reported, but they relate to homogeneous, solution-based systems. See: Ameerunisha, 1995; Effing, 1995; Kimura, 1996, Kimura, 1994; Shinkai, 1981; Shinkai, 1982; Takeshita, 1998; and Winkler, 1989. These reports do not involve redox-active moieties attached to a surface.
Photoswitching of membrane permeability has been reported. See: Anzai, 1994; Aoyama, 1990; Fujinami, 1993; Hauenstein, 1990; Kumano, 1983; Okahata, 1984; Schultz, 1977.
These applications are directed toward extraction or separation, but they do not involve affixing selective, switchable binding moieties to a surface. Instead, they involve mediated solution transport through a membrane. They also are not semiconductor-based.
Semiconductor-mediated photoreduction of species out of aqueous solution, particularly metal ions, is well-known in the art. See, for example, Borgarello, 1985; Borgarello, 1986; Brown, 1985; Chenthamarakshan, 1999; Curran, 1985; Eliet, 1998; Herrmann, 1988; Jacobs, 1989; Serpone, 1987; and Tanaka, 1986. Photoreduction of metal ions onto semiconductor surfaces has been proposed as technique for extraction of metal from solution (e.g., Borgarello, 1985; Borgarello, 1986; and Herrmann, 1988). A major obstacle in the practical application of such methods has been economical removal of the metal and regeneration of the semiconductor surface. Photoreduction has also been employed for photodepositing metals, e.g., in patterns onto semiconductor substrates, and is one technique for so-called “electroless deposition”. This technique has been applied where deposition is intended to be permanent.
Ionic dyes have also been photoreduced in colloidal semiconductor suspensions. See, for example, Brown, 1984; Yoneyama, 1972; Gopidas, 1989; and Vinodgopal, 1994. However, none of the systems described is switchable, and none involves a redox-active moiety adsorbed or bound to a solid.
One semiconductor-based light-driven switchable system for dissolution of metal ions has been described which is based on the reversible photoreduction of aqueous cupric ion (Cu++) to an insoluble cuprous (Cu+) complex on the semiconductor. See: U.S. Pat. No. 5,332,508; Foster, 1993 and Foster, 1995. The Cu+ complex reoxidizes on exposure to air, with return of Cu++ to solution. Photoreduction apparently must be carried out in the absence of oxygen. Further, an organic co-solvent is apparently required as a “hole scavenger.” Photoreduction does not occur in simple aqueous solution, evidently because the co-solvent acts as a chelating agent for Cu+. Organic contaminants or added organics in treated solutions are photooxidized. This system does not employ a redox-active moiety adsorbed or bound the semiconductor.
Redox-switched extraction by electrochemically controlled reversible intercalation and de-intercalation into redox-active crystals on functionalized electrode surfaces has been reported for lithium ion recovery (U.S. Pat. No. 5,198,081 and Kanoh, 1993) and Cesium ion separation (Lilga, 1999). These applications do not involve surface immobilization of particular redox-active binding agents and are never photodriven. Photodriven insertion of dissolved species into crystal lattices has been reported See, for example, Betz, 1985; Betz, 1984; Betz, J. Appl. Electrochem., 1984; Tributsch, 1980; and Tributsch, 1983. These reports do not involve surface binding of redox-active moieties and protons, rather than metal ions, are the intercalating species.
Redox-switchable materials also have potential application in electrosorption; data storage; and light energy storage. “Electrosorption”, the extraction of ions from aqueous solution by electric fields, is conventionally accomplished by electrostatic adsorption of counterions to electrodes surfaces (Johnson, 1971). Recent interest focuses on high-surface-area electrodes such as carbon aerogels (Farmer, 1996). Redox-active electrodes have been mentioned only in the context of “pseudocapacitance” for boosting the capacity of so-called “ultracapacitors” (Conway, 1997). Redox-swichable materials have apparently not as yet been applied in such applications.
The use of photoactive molecules for bitwise storage in computer memories have been reported. See, for example, Feringa, 1993; Tsivgoulis, 1995; and Willner, 1997.
Solar energy is expected to become a major primary energy source in the next few decades. Its disadvantages are well known, as it is both diffuse and intermittent. An attractive approach to solving these problems is the one already taken by plants: trapping the energy of sunlight into chemical bonds. Such “artificial photosynthesis”, the use of sunlight to produce fuel, has a literature stretching back to the early 1970s (e.g., Fukujima & Honda, 1972; Bard & Fox, 1995; Bolton, 1996) although it is not yet practical. Photo-switchable redox active materials in which the energy of the absorbed photons can be trapped in a chemical form can be applied to solar energay storage.
Photoactive semiconductors such as Si and GaAs functionalized with ferrocene moieties have been proposed for applications in artificial photosynthesis. See: Bocarsly, 1980; Bolts, 1978; Bolts, 1979; Bolts, 1979, pp. 1378-85; Gronet, 1983; Kashiwagi, 1998; Legg, 1977; and Tatistcheff, 1995. The functionalization was employed to shield the semiconductor from corrosion by the aqueous solution. In contrast to oxide semiconductors, Si and GaAs semiconductors readily photooxidize in water. Ferrocene is not used as an energy-storage couple in these applications.
There is a significant need in the art for redox-switchable materials for the various applications noted above and in particular for photo-driven or activated redox-switchable materials. The present invention provides improved solid phase redox-switchable materials.